As has been indicated on this page , the Top Boost circuit was originally designed by JMI for its new AC30 Super Twin amplifiers, announced somewhat elliptically in the late summer of 1961 as the "Stereophonic" range.

At first the circuit took the form of an "add-on" unit comprising of new treble and bass controls and the circuitry and assembly to implement them, fitted at factory to batches of Super Twins and, at request, to standard Twins. The assembly was supplied as a kit from 1963 for users or repair shops to retro-fit to existing AC30s.

Below, a short list of AC30s from 1962 with Top Boost circuitry added or rather incorporated early on, controls on the back panel:

4995 - Twin, the Top Boost bracket has two fixing points at top (normally there is only one), no component date codes visible though the electronics are evidently old (and factory installed).

5311 - Super Twin, perhaps a replacement trafolyte facing panel and control knobs. The electronics are old (and probably factory installed).

5329 - Super Twin, Hunts cap. visible but no component date codes. Perhaps sold with Top Boost already fitted.

5648 - Super Twin, the white trafolyte facing panel is now missing, and the foot of the bracket looks to have been shortened. The ECC83 has the date code "JL" = November 1962. The Hunts cap. has "YIW" = 3rd week of 1961.

5726 - the Hunts cap. has the date code "HIH" = 23rd week of 1962. Perhaps sold with Top Boost already fitted.

It is perhaps worth adding that a chassis from late 1962, sold recently on its own (no cabinet, no speakers), seems to have gone back to the factory to have Top Boost added in 1965 (pots with January '65 date codes) - either that, or a repairman installed it from a JMI kit.|

Serial number 5329 amplifier section, the top boost unit incorporated (as is normal) in a cut-out in the back panel.

Detail from the Vox "Precision in Sound" newspaper-format catalogue, September 1964. The Top Boost unit kit. Note the "pseudo stereo effects", a slight stepping back from the "stereophonic" of 1961 and 1962.

The circuit diagram

The circuit diagram, JMI OS/010, was first drawn out formally by John Bell on 12th December 1961. Two days earlier he had redrawn the sheet for the AC2 - probably to assist Triumph Electronics in getting production underway - see this page. At any rate, below, a detail of OS/010 showing the Top Boost circuit itself: treble and bass controls - 1M log potentiometers, treble at left, bass right - and associated network.

Detail of JMI OS/010. The hand-written notes of capacitor and resistor numbers were added later.

As Glen Lambert indicated many years ago, the new Top Boost was simply a "lift" from a circuit adopted by Gibson in the mid 1950s for its Vanguard GA-77 amplifier:

Detail of the circuit diagram for the Gibson Vanguard GA-77 amplifier, dated 1953/1954. Where the pots are concerned "Audio" indicates "logarithmic" or "log" taper.

Quite how Dick Denney encountered the GA-77 is unknown. But as an active musician, he will have seen good numbers of amps on stage belonging to members of bands other than his. Evidently the GA-77 caught his eye and ear.

Where the AC30 is concerned, the Top Boost controls give a greatly enhanced tonal range - far greater at any rate than that offered by the simple tone cut conrtol of the original circuit. One drawback of the Top Boost controls however is that they are highly interactive. Settings made too quickly can often result in over-brightness and what can only be decribed as a sort of "waxiness" to the sound.

Different issues of the circuit diagram

(1) - as originally drawn by John Bell in December 1961. No copy has come to light so far.

(2) - issue 2, June 1963, but with no changes to the circuit noted. There is no text within the box at top right immediately under the diagram - see (3) below. The only voltage noted is: "HT 230v" at the top of the main diagram. Copies of this sheet were evidently sent out in wallets of circuit diagrams issued by JMI.

(3) - issue 2, June 1963, a version of the above. However, within the box at top right there is a line of text underneath the inset diagram: "MODS TO AMPLIFIER AC30/6. DRAWING NO. A/026. This was the sheet embodied in the Thomas Organ Vox Amplifiers Service Manual.

Detail of the sheet in the Thomas Organ Manual: issue 2, June 1963, but there is no note of what the changes in issue 2 were. The text in purple does not appear in (2) above.

(4) - issue 3, February 1965, the one currently in circulation on the web. Voltages are given throughout - the main HT is now noted as being 320v - and valve pin numbers are provided. Jim Elyea gives a print of an uncirculated copy in his schematics portfolio (sheet number 57). In issue 3, the additions made in issue 2 are listed as being voltages and valve pin numbers.

Presumably there was some version of issue 2 that gave the spread of voltages and the valve pin numbers as encompassed in issue 3.

A detail from the information panel of issue 3 listing what the changes in issue 2 were (or should have been).

(2) OS/010 from the wallet pictured below. The HT voltage is given as 230v. An addition of 19th June, 1963, is signalled, but there is no note of what it was. There is no legend inside the panel, top right.

A JMI wallet of circuit diagrams, one of five currently known. These were issued by JMI to its engineers and sometimes sent out to repair shops. Twelve sheets survive in this one.

(3) Thomas Organ, sent to the States in late 1964. "OS/010" is re-written; HT voltage is 230v; reference to the AC30/6 and circuit diagram A/026 has been added at the foot of the panel, top right.

(4) The version common on the web. HT voltage is given as 320v. The substance of the addition of 19th June, 1963, is itemised" "Voltages and Pin Nos added". The legend "Vox Top Boost Mod" was added in the 1990s (the identity of the writer is known).

Later uses

That the Top Boost circuit encapsulated a sound approved of by Dick can be seen in the preamps of the AC50 and AC80/100, designed in parallel in mid 1963, Derek Underdown being responsible for the power sections, Dick the "front ends". And these "Top Boost" preamps made their way through to later revisions of the two models. A page on AC30s with Top Boost controls integrated in the control panel will be available soon.

Detail of the Top Boost circuit (as above).

Detail of JMI OS/044, the circuit diagram for the AC50 mark 1 (two inputs, thin-edged box, as issued to The Beatles et al.)

On to the next page: surviving AC30 Top Boost units.